The present invention relates to a simultaneous biaxial stretching machine, comprising a gripping means return line system on each side thereof with a guide rail for the gripping means, a return screw with regressive lead and a gripper input means to feed the gripping means to the regular working line of the machine.
The application for which the present invention is intended is the simultaneous longitudinal and transversal stretching of thermoplastic sheets or films.
A simultaneous biaxial stretching machine of the type under consideration has been disclosed in the East German Democratic Republic Pat. No. 23,367 and in the Austrian Pat. No. 243,498, respectively. With each of these machines the gripping means are positively transported by transporting screws, chains feeding wheels etc., for it has been found that such positive transportation and handling of gripping means is an absolute necessity for obtaining perfect operational results. Within range of the stretching line, the gripping means are progressively accelerated with the aid of a transport screw with progressive lead corresponding to the longitudinal stretch ratio concerned, while in the return line the said gripping means are progressively decelerated with the aid of a return screw with regressive lead, for it is not permissible that the gripping means run freely such as on an inclined path because the heavy gripping means might be damaged. The mass of a gripping means is in the range from a few 100 grams up to a few kilograms.
Since it is necessary within such a machine to transport said gripping means in different directions and at varying speeds (including accelerations and decelerations) gripper transport equipment of different types must be used, e.g. transport screws. transport chains and feeding pulleys or wheels. To ensure proper guiding of the gripping means throughout the entire machine, it is a factor of primary importance that speed and the leads of the various transporting facilities be very carefully tuned to each other.